Atmospheric ozone concentration detector

ABSTRACT

An ozone detector in the form of two basic embodiments which can measure ozone concentration in ambient air at 0.001-1 ppm. The first embodiment includes a stretched elastic material and a standard which can give ozone concentration in response to degree of microcracking or frosting of the elastic material. The second embodiment includes a chamber containing a long strip of elastic material, a component for stretching the material, a light source for generating a light and illuminating the film and a detector for detecting reflected, transmitted through or scattered light by the material.

CROSS-REFERENCE TO RELATED APPLICATION

This is a continuation-in-part application of application NC 77,117bearing Ser. No. 08/625,506 entitled “Atmospheric Ozone ConcentrationDetector” which was filed Mar. 29, 1996, now U.S. Pat. No. 5,972,714 andwhich is herein incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of Invention

This invention pertains to measurement of ozone.

2. Description of the Related Art

Ozone is deleterious to materials and to humans. OSHA's limits foraverage ozone concentration are up to 0.1 ppm over 8 hours or up to 0.3ppm over 15 minutes.

Presently, there are a number of techniques for measuring atmosphericozone concentration. One technique employs ultraviolet light absorptionwhereas another employs the differential creep of rubber technique.

The technique of ultraviolet light absorption takes advantage of a 254nm absorption line of ozone in the electromagnetic spectra and thusmeasures the concentration of ozone directly. Here, a measured sample ofair is pumped into a chamber and illuminated at one end with a lowpressure cold cathode mercury vapor ultraviolet light. The ultravioletlight from the mercury lamp has emission at 254 nm. At the opposite endof the chamber is a cesium telluride vacuum diode detector. Thedetermination of ozone is carried out in two steps. Initially, anozone-free reference gas sample is pumped into the chamber and thetransmitted light intensity is measured. Any ozone present in thereference gas is rapidly destroyed by passing the gas over manganesedioxide. In the second step, an “ozone gas sample” is pumped into thesame chamber and the transmitted light is measured. The ozoneconcentration in the “ozone gas sample” can be easily determined byapplication of a formula.

Ozone detectors which operate on the basis of ultraviolet lightabsorption can detect as little as about 0.001 ppm of ozone but have thedisadvantage of being somewhat large at about 19″×12″×6.5″; of beingheavy at about 22 pounds; of requiring a full line voltage of 115V; ofrequiring a warmup time of about 2 hours; of being expensive at about$4,500-$12,000 per detector; and of requiring to be stationary. Inshort, such detectors are sensitive, expensive and are intended forstationary laboratory use.

The fact that a detector must remain in the lab is a seriousdisadvantage because ozone concentration often needs to be measured inwidely separated locations, such as when one is determining the averageozone concentration over an entire city or when one needs to measure theambient ozone in every room in a building. Furthermore, another criticaldisadvantage of an absorption ozone detector resides in the fact thatozone is very chemically active and thus easily destroyed inside manycontainers, which precludes sample collection.

In a differential creep of rubber technique, a standard rubber thread isdivided in half, one part is exposed to the atmospheric ozone whereas,the other part is protected from ozone. The unexposed portion creeps ata lower rate than the exposed portion and pulls an indicating needleattached to the exposed rubber along a scale, thus giving a measure ofozone concentration.

SUMMARY OF THE INVENTION

An object of this invention is an apparatus for measuring ozone.

Another object of this invention is a quick measurement of ozone inatmosphere by means of a stretched elastomer which frosts over inresponse to microcracks created in the elastomer by ozone in theatmosphere.

Another object of this invention is a highly practical and a highlysensitive electrical measurement of light intensity from a frostedelastomer adjusted for light intensity from unfrosted elastomer.

Another object of this invention is a device for providing rudimentary“go-no go” ozone concentration results.

These and other objects of this invention are realized by a structurecharacterized by a stretched or stressed elastic material which becomeswhitened or frosted or transluscent when ozone in the surrounding aircontacts it and creates microcracks therein which scatter light.

BRIEF DESCRIPTION OF THE DRAWING

For a better understanding of these and other objects of the presentinvention, reference herein is to the following detailed description ofthe invention which is to be read in conjunction with the accompanyingdrawings, wherein:

FIG. 1 is a front elevation view of the go-no go ozone detector showingan elastomeric material which is stretched in an environment containingozone to produce a degree of frosting which is compared to a standard.

FIG. 2 is a front elevation of an ozone detector containing a lightsource, an extended elastic material and means for detecting lightintensity of light reflected from the material and converting it to anelectrical value.

FIG. 3 is a schematic of experimental set-up which was used to obtaindata on elastic material samples.

FIG. 4 is a graph of relative transmitted light intensity throughelastic films versus time at constant tensile stress for three ozoneconcentrations.

FIG. 5 is a graph of light intensity versus time for elastic filmscarried out at the same ozone concentration but at two different tensilestresses.

FIG. 6 is a graph of intensity versus time carried out at the sametensile stress and about the same ozone concentration but on two filmshaving different Young's modulus.

FIG. 7 shows measurement of backscattered light resulting from directinglight at a stretched elastic material exposed to ozone-containing air.

FIG. 8 is an illustration of a trapezoidal rubber sample of uniformthickness but having non-uniform cross-sectional area which is stretchedand then exposed to an ozone-containing air sample.

DETAILED DESCRIPTION OF THE INVENTION

The subject matter herein is directed to measurement of ozone by meansof a stretched elastic material which frosts when exposed to ozone. Theozone detector that measures ozone concentration in the surroundingenvironment operates on the principle of ozone creating a multitude ofmicrocracks in the stretched elastic material.

Cracks produced in an elastic material by ozone extend generallyperpendicularly to the direction of the stretching force. The cracks areon the order of 1 micron to 1 mm long and about 1 mm or less in width.Length of the cracks is variable and increases with ozone concentrationand duration of exposure whereas the crack width remains essentiallyunchanged.

Stress or strain has two effects on the ozone-induced cracking ofstretched rubber. Below a certain threshold strain of about 1 or 2%, noozone cracking takes place. Beyond this minimum threshold, thesensitivity to ozone-induced frosting increases with increasing strain.Hence, the concentration of cracks increases so that higher strainsaccelerate development of opacity. In using a stretched rubber to detectozone, higher strains confer higher sensitivity, and thus the ability todetect low levels of ozone at shorter times. Lower strains reduce thesensitivity, and thus increase the working time, whereby ozone can bedetected.

An entire range of sensitivities can be obtained by using elasticmaterial with a non-uniform cross-sectional area. The ends of an elasticmaterial of variable thickness are displaced a distance corresponding tosome average strain, as shown in FIG. 8. Since the force along thestretched direction is constant, the stress, which is equal to the ratioof the force to the cross-sectional area, varies from the top, where itis a minimum, to the bottom, where it is maximum. The strain, to a verygood approximation, is proportional to the stress, so that the strainalso varies from some minimum to some maximum value. The low strain endof the sample is below the threshold strain for ozone cracking. As aconsequence of the geometrically-induced distribution of strains, theozone response varies from zero at the top, where the cross-sectionalarea is thickest, to very high at the bottom, where it is thinnest.Opacity will develop initially at the bottom, and over time propagateupwards. It will stop when the strain falls below the minimum necessaryto induce ozone stress cracking. Through the use of a predeterminedcalibration, the measurement of opacity or its reciprocal, i.e., lighttransmission, at any position provides a measure of the atmosphericozone concentrations.

In the first or the “go-no go” embodiment, an elastic film is stretchedon a frame and the stretched film is exposed to ozone-containingenvironment which causes microcracks. The microcracks appear to thecasual observer as “frosting” in which the elastic material istransformed from clear to transluscent. The frosted film is thencompared to a standard to determine the ozone concentration.

In the second embodiment, light is projected onto a stretched elasticmaterial disposed in an ozone-containing environment and its scatteredor reflected intensity is measured on the opposite side or on the sameside. This intensity is compared to a base intensity, then converted toan electrical signal and the relative intensity is then calculated. Therelative intensity is then converted to a read-out of actual ozoneconcentration in the surrounding atmosphere.

The elastic material used in the first or the second embodiments, or anyembodiment of this invention, can have a uniform or non-uniformthickness or cross-sectional area.

The first embodiment, which can measure ozone concentration from a lowof about 1 ppb to a high of about 500 ppb, is illustrated in FIG. 1.This embodiment is also referred to herein as “go-no go” since it cangive ozone concentration quickly in a matter of a few minutes or less.In this embodiment, elastic material 10 is secured at both ends toholders 12, 14 and then stretched on frame 16 which is rectangular andconsists of rigid members 18, 20, 22, 24.

The frame can be of any suitable form. In operative condition, holders12, 14, are disposed respectively on frame members 18 and 22 and elasticmaterial 10 is stretched between members 18, 22 since the distancebetween members 18, 22 is greater than the length of material 10 in itsunstretched condition. The length of the frame can be changed to providefor different stress in the elastic material in order to adjust theozone sensitivity. The ozone sensitivity can be increased by stretchingthe elastic material to a greater degree.

The material stretched on the frame secured to holders 12, 14 atopposite ends thereof is placed in ozone-containing environment for apredetermined period of time to allow ozone to attack it and to developmicrocracks therein which results in a frosted material. The frostedmaterial 10 is compared to a standard 26 that can be in the form of acard containing a series of indicia marked 0, 10, 20, 40 and so on ofdifferent intensity of frosting, with darker colors corresponding togreater concentrations of ozone in the atmosphere.

The embodiment illustrated in FIG. 1 is suitable for measuringconcentration of ozone at many locations along a highway, over a city orat any other location. This embodiment can quickly give a measurement inany location far away from a laboratory.

FIG. 2 illustrates the second embodiment of ozone determination whichcan routinely measure ozone concentration. There is no fundamentallimitation on the range of ozone concentration, measurable with thisembodiment, however, for purposes of specificity, this embodiment canmeasure ozone concentration, from a low of about 1 ppb to a high ofabout 1000 ppb. The measurable zone concentration can be furtherextended by modifications in the elastic material, number of the elasticmaterial layers, intensity of the light, sensitivity of the detector andother parameters that should be obvious to a person skilled in the art.

In the second embodiment, elastic material 50 is rolled up on spool 52in airtight chamber 54 where substance 56, such as manganese dioxide,may be included in the chamber to neutralize deleterious effects ofozone on the material. Airtight condition of chamber 54 can be assuredby providing clamp 56 which provides an airtight seal at opening 58through which material 50 exits chamber 54. In this arrangement,material 50 is clamped against the chamber walls to give an airtightseal around opening 58 and substance 57 is present in chamber 54 toprevent ozone from attacking the material. Therefore, material 50 inchamber 54 should be free of frosting due to ozone.

Material 50 is drawn from chamber 54 through opening 58 onto take-uproll 60 directly above spool 52 and spaced therefrom. A predeterminedand adjustable tension is applied to the material by means of calibratedspring acting on the take-up roll 60. Auxiliary off-set pulley 62,riding on the periphery of take-up roll 60, can also provide sufficienttension in the material. A minimum tension in the material is necessaryin order to determine ozone concentration in the surrounding gaseousatmosphere. The minimum tension required in the material in order todevelop at least some frosting therein will depend on many parameterssuch as the type of polymer material used and stiffness of the polymer.However, such ,minimum tension contemplated herein is typically at leastabout 5 kPa, more often at least about 50 kPa.

At some convenient point between spool 52 and take-up roll 60, buttypically close to opening 58 in order to avoid exposing material 50 toan atmosphere containing ozone, light source 63 is positioned close tothe material. Any light source may be used. Typically, the light sourceis a light emitting diode (LED) or a laser. A lens, not shown, can beinterposed between the light source and the material to enlarge diameterof the light impinging on the material. The light emitted by lightsource 63 is defined by diverging lines 64, 66 which impinge onconverging lens 67 and exits lens 67 as light defined by converginglines 68, 70.

Light emitted by light source 63 before passing through the material 50has intensity I₀ which is greater than transmission intensity I of thelight after passing through the material. Depending on amount of ozonein the air surrounding embodiment of FIG. 2, the material will befrosted with microcracks therein which will scatter or diffuse lightpassing through the material. With greater concentration of ozone in thesurrounding air, the elastomer will be even more frosted and intensityof the light transmitted through the material will be lower. Therefore,greater concentration of ozone in the surrounding air will create moremicrocracks in the material or a greater degree of frosting which inturn, will reduce intensity of the light passing through the materialexposed to the ozone-containing atmosphere. Thus, intensity of lightpassing through the material is proportional to frosting of the materialwhich is proportional to the quantity of ozone in the surrounding air.By detecting intensity of the light passing through the material, it ispossible to correlate quantity of ozone in the surrounding air with theintensity of the light passing through the material.

In FIG. 2 light detector 72 measures light intensity of the lightpassing, through the material which is then compared to base intensityand the difference converted to a current. The current is then amplifiedin amplifier 74 and then converted to a voltage and read as voltage bymeans of voltmeter 76, as desired, and finally correlated to ozoneconcentration in the surrounding air.

The embodiment of FIG. 2 is operated by drawing a fresh material fromthe spool between the light source and the light detector and applyingtension so that the material is stretched. Immediately before or afterdrawing the material and before ozone is allowed to create surfacecracks therein, a baseline transmitted light intensity I₀ is measured bypassing light through the stretched material. The material is thenexposed to the ozone-containing atmosphere for a given length of time,which will depend on the ozone concentration and other circumstances.The transmitted light intensity I is measured once again. Alternatively,the rate of change of the ozone concentration can then be readilydetermined from the ratio I/I₀ or the rate of change of I/I₀ by means ofa calibration.

FIG. 7 shows light source 710 producing light 712 of certain intensityimpinging on sample of horizontally disposed elastic material 714 atabout the angle of θ. Incident light 712 reflects light 716 from thematerial and backscatters at an angle to the material scattered light718. Intensity of reflected light 716 is smaller than intensity of light712 and intensity of backscattered light 718 is also smaller thanintensity of light 712. Light detector 720, disposed on the same side ofthe material as light source 710, can be made to sense integral lightintensity backscattered, or reflected light 718. Integral lightintensity is the cumulative number of photons scattered or reflectedfrom an area of the sample and collected over a finite angular range fordetection.

Referring specifically to FIG. 8 shows a piece of rubber elasticmaterial 810 having uniform thickness but non-uniform cross-sectionalarea that is stretched. The rubber piece 810 is shown in front view “A”as a trapezoid having upper edge 812, lower edge 814, and two side edges816, 818. Upper edge 812 is longer than lower edge 814 and the two sideedges 816, 818 are of equal length, although they, can be of differentlength. The upper and lower edges can be parallel to each other but theyneed not be. As shown in view “B” of FIG. 8, thickness of rubber piece810 is uniform, although it can be variable. The edges are shown asbeing straight, although the edges can be of a different shape.

For determination of presence of ozone or ozone concentration, upper andlower edges 812 and 814 are clamped and the rubber piece 810 isstretched by drawing the clamps further away from each other to impartstress to the rubber piece 810. As stretching proceeds, rubber piece 810becomes distended and since upper edge thereof is longer than its loweredge, the rubber piece becomes thinner at the bottom than at the top.View “C” of FIG. 8, is the front view of the rubber piece 810 afterstretching and view “B” is its side view. As shown in view “C”, rubberpiece 810 has retained its general form after stretching althoughdistended, with upper edge 812 generally parallel to lower edge 814, andside edges 816, 818 generally of equal length. As shown in view “D” ofFIG. 8, rubber piece 810 is non-uniformly stretched least at the top,intermediate at about the middle, and most at the bottom. Stretching ofrubber piece 810 is inversely proportional to the cross-sectional areathereof—with the largest cross-sectional area at the top of the rubberpiece, intermediate cross-sectional area at about the middle, and thesmallest cross-sectional area at the bottom.

View “B” of FIG. 8 shows uniform thickness of the rubber piece before itis stretched whereas view “D” of FIG. 8 shows a non-uniform thickness ofthe rubber piece after stretching being thicker at the top and thinnerat the bottom. Generally speaking, thickness of rubber piece 810 in view“B” of FIG. 8 is thicker than that of rubber piece 810 in view “D” ofFIG. 8. Initial thickness of the rubber piece before stretching can bemade to be variable, as desired.

The embodiment illustrated in FIG. 8 has particular utility insituations where ozone concentration in a space is not known and it isdesired to use an elastic material sample of varying thickness afterdistention varying from thinnest to thickest.

Selection of a suitable elastic material is made on the basis of whetherozone deleteriously affects any physical property thereof which resultsfrom chain scission caused by ozone attack on the material. This can bedetermined by measuring a physical property of the material, such asstiffness, before and after exposure thereof to ozone. Suitable materialis cross-linked and flexible and can be stretched on application of aforce thereto and elongated. Upon removal of the force which stretchesthe material, the material returns to a position which can coincide withits original extent or only part way. The term “elastic material” neednot conform to the classical definition of an “elastomer” although itcan. A classical definition of the term “elastomer” is any substancehaving properties of natural, reclaimed, vulcanized or synthetic rubberthat stretches markedly under tension, has a high tensile strength inexcess of about 5000 g/cm², retracts rapidly, and recovers most of itsoriginal dimensions.

Typical elastomers that are believed to be suitable herein includepolyisoprene, polychloroprene, polybutadiene, styrene-butadiene, butylrubber, nitrile rubber, ethylene-propylene-diene copolymers, and manyothers. Since ozone can attack both saturated and unsaturated bonds andcause chain scission, suitable elastomers need not contain unsaturation.For high ozone sensitivity, unsaturated elastomers are preferred.

Different elastomers are known to have different ozone concentrationsensitivity. Typical elastomers suitable herein are those materialswhich are classical elastomers, preferably natural and synthetic rubberswhich are characterized by presence of unsaturation, and especiallypolybutadiene or natural rubber, which have good ozone sensitivity. Thismeans that stretched polybutadiene and natural rubber readily frost whenexposed to an atmosphere containing ozone.

Thickness of the elastic material is only relevant in the context ofextensibility thereof and passage of light there-through. Based onpractical considerations, thickness of a material either to be used infirst or second embodiments, or in another embodiment based on theprinciples disclosed herein, is typically less than about 10 mm, moretypically in the approximate range of 0.03-2 mm. With multiple layers ofthe material, substantially thinner than 0.03 mm films of the materialcan be used.

The elastic material is typically optically clear and has littleabsorbance of less than about 10%. As more colorant, such as carbon, isused in the elastic material, its absorbance increases and lighttransmission decreases. As little as a few percent of carbon additive inthe elastic material can render the elastic material non-transparent tolight.

Stretching of the material is prerequisite to the proper determinationof ozone concentration in air. There is a certain minimum tension belowwhich the material will not frost when exposed to any ozoneconcentration, even a high ozone concentration, such as about 0.5 ppm.This minimum tension is different for different materials. Forpolybutadiene, a useful minimum strain is believed to be about 2% andfor natural rubber it is believed to be about 4%. At the opposite end,the maximum limit is that extension which will result in failure orrupture of the material. Therefore, the material can be stretchedbetween these limits, bearing in mind that a more highly stretchedmaterial will generally be more sensitive. Typically, during the courseof ozone determination as disclosed herein, the stress of the materialwill typically be, in the approximate range of 10-5,000 kPa, moretypically 50-2,000 kPa.

The method for detecting ozone concentration comprises the steps ofstretching an elastic material; impinging a gaseous mixture containingozone upon the material and allowing frosting of the material to takeplace based on formation of microcracks due to ozone in the gaseousmixture; and detecting ozone concentration in the gaseous mixture bymeans of light transmission through the microcracked elastic material orreflected light from the elastic material or scattered light, based ondegree of frosting of the material which is directly related to theozone concentration in the gaseous mixture. The step of impinging lightthrough the gas mixture containing ozone is accomplished by projecting alight beam therethrough emanating from a light source at the material.

Having described the invention, the following examples are given asparticular embodiments thereof and to demonstrate the practice andadvantages thereof. It is understood that the examples are given by wayof illustration and are not intended to limit the specification or theclaims in any manner.

EXAMPLE 1

This example demonstrates the use of elastic material of a uniformcross-sectional area and of a uniform thickness before stretching todetermine ozone concentration.

The elastic material samples used herein were Goodyear Tire and Rubbertype 1209 polymeric 1,4—polybutadiene mixed with dicumyl peroxidecrosslinking agent prepared by compression molding for 30 minutes at160° C. Samples I were prepared with 0.50 parts dicumyl peroxide per 100parts of rubber (phr) and samples II were prepared with 0.94 phr dicumylperoxide and therefore, were much stiffer than samples I. The samplefilms were 65 mm×13 mm×1.6 mm.

FIG. 3 shows a schematic of the experimental apparatus used to obtainthe data herein. Each sample 100 was mounted in environmental chamber102 made from 12 mm uniformly thick sheets of clear poly methylmethacrylate. Chamber 102 consisted of top panel 104, bottom panel 106,and side panels 108. The chamber measured about 17 cm×17 cm×36 cm.Sample 100 was clipped at its upper end to a rigid bar 110 and at islower end to a clip 112 which in turn was connected to a wire 114 andthat in turn was connected to a ring-like holder 116 on which wereplaced weights 118. The weights were varied as desired. Ambient room airwas admitted into chamber 102 through entry port 120 at the top of thechamber and ozone was measured by taking air sample from the chamberthrough exit port 122 located at the bottom of the chamber. Measurementof ozone was made by drawing a known volume of air from chamber 102through exit port 122 and through Drager tubes (not shown) availablefrom National Drager, Inc., as item No. 6733181, with a calibrated handpump (not shown), also available from National Drager, Inc., as model31. For lowest concentrations of ozone in the chamber, sensitivity ofthe Drager tubes was increased by increasing the pumped volume of air.

The Tesla coil 124 was actuated periodically to produce sparks 126 whichproduced ozone in chamber 102. Increasing frequency of sparks 126increased ozone concentration of ozone in the chamber whereas decreasingfrequency of sparking had the opposite effect. One-half hour was allowedfor ozone concentration to reach equilibrium after frequency of sparkingwas changed although ozone reading showed essentially constant valueafter only the first 10 minutes. During the equilibration period beforethe sample was stressed, intensity of light passing through the samplewas monitored until no change was observed.

The light source was Uniphase 1104 HeNe laser 128 which produced a lightbeam 130 of 632.8 nm wavelength which was passed through spreading lens132 positioned in front of the light source. The purpose of lens 132 wasto spread or enlarge diameter of the light beam 130 emanating from lightsource 128 to where the beam diameter 134 on sample 100 was 1 cm.Scattering of the light through the walls of the chamber was negligible.The light beam 130 from the light source passed through spreading lens132, through chamber sidewall 108, through film sample 100, through theopposite chamber sidewall 108, through converging lens 136 and intolight detector 138 which was a photodiode detector UV100 from EG & GInc. Current from the light beam entering light detector 138 wasamplified by current amplifier 140 and the transmitted light intensityof the light beam passing through the film sample was obtained fromvoltmeter 142.

Just before sample 100 was stretched in chamber 102, a baseline lightintensity was recorded. The relative high transmittance through thesample reported herein was determined by dividing the measured intensityby the baseline intensity.

After the ozone concentration in chamber 102 reached equilibrium afterone-half hour of steady state operation, the sample was stressed byplacing calibrated weights 118 on holder 116 and immediately initiallight intensity was recorded followed by light intensity recordationthereafter at 30 second intervals. Because the samples had manyinhomogeneities that scatter light, such as trapped dust particles, theinitial relative transmitted intensity of the sample was usuallydifferent from the unstretched sample. The inhomogeneities in thesamples increased noise of the experiment which was reduced by lens 132.Lens 132 increased the illuminated area and thus averaged out theinhomogeneities. It is estimated that the inhomogeneities contributed anerror of about 5% in the value of the transmitted light.

Results of the experiments are illustrated in FIGS. 4, 5 and 6. FIG. 4shows three plots of Time in minutes versus Relative TransmittedIntensity at a constant stress (σ) of 850 kPa (9,5600 g/cm²) for threelevels of ozone concentration in the chamber: 0.035 ppm (35 ppb), 0.08ppm (80 ppb) and 0.3 ppm (300 ppb). Accuracy bf the ozone concentrationsin the chamber is estimated to be ±20%. On this basis, it is estimatedthat there may be ±7 ppb error in the 35 ppb ozone concentration, ±16ppb in the 80 ppb level and ±60 ppb in the 300 ppb level. The resultsdemonstrate that as the ozone concentration was increased, the rate ofloss of the relative transmitted light intensity, or the frosting rate,increased dramatically. The scatter of the data at very short times isattributed to inhomogeneities. Even at the lower ozone concentration of35 ppb, the frosting rate was still appreciable, such that 50% of theinitial light transmittance was observed after 18 minutes. This ozoneconcentration is about ⅓ of the OSHA 8 hour average work limit of 100ppb and is below ambient conditions present in most US cities on mostsummer days.

The effect on light intensity of increasing stress from 426 kPa (4,250g/cm²) to 850 kPa (8,500 g/cm²) at constant ozone concentration of 35ppb using samples I is given in FIG. 5. The figure shows that at the lowstress of 426 kPa, the frosting rate was low but not zero and at thehigh stress of 850 kPa, the frosting rate increased dramatically. Thus,sensitivity of the sample film can be tuned to fit the needs of thesituation; if a long term average is desired, the applied stress shouldbe low, thereby increasing the effective life of the sample. If, on theother hand, a short term average is desired, the applied stress can behigh in order to obtain a quick accurate reading of ozone concentration.

The effect of changing modulus E of the samples at constant engineeringstress of 450 kPa (4,500 g/cm²) and a nearly constant ozoneconcentration of 380 v. 300 ppb is shown in FIG. 6. The samplestypically become less flexible on exposure to ozone. The modulus E ofsamples II was about one-half of samples I, i.e., 1.25 v. 2.51 Mpa,meaning that samples II were about twice as stiff as samples I. FIG. 6shows that for the stiffer samples II, sensitivity to ozone decreased.For samples II, there remained a low but measurable loss of relativetransmitted light intensity. Thus, if it is desired to adjustsensitivity over a wide range, beyond what can be provided by adjustingthe stress, this can be achieved by changing to a sample with anappropriate modulus.

Modulus E, referred to herein is Young's Modulus which was determined bydividing the true stress σ by the extension in the vicinity of 20%strain.

Reference herein is made to true stress and engineering stress. Forpurposes of clarification, true stress is the force or the weightapplied to the sample divided by the stressed area of the sample whereasengineering stress is the force or the weight applied to the sampledivided by the unstressed area of the sample.

EXAMPLE 2

This example demonstrates the use of elastic material of a non-uniformcross-sectional area but of a uniform thickness before stretching todetermine ozone concentration.

As shown in FIG. 8, rubber elastic material 810 had uniform thicknessbut non-uniform cross-sectional area that was stretched. Rubber piece810 shown in front view “A” had a trapezoidal shape with upper edge 812that was 4 cm long, and lower edge 814 that was 1 cm long. Verticaldistance between the upper and the lower edges was 6.5 cm. The upper andlower edges were parallel to each other. Thickness of rubber piece 810was a uniform 0.15 cm from top to bottom.

Rubber piece 810 was clamped at its upper and lower edges and thenstretched to 1.76 times its length to where its vertical distance was11.26 cm. After stretching, the upper edge was still 4 cm, the loweredge was reduced to 0.5 cm, thickness of the rubber piece at its upperedge was still 0.15 cm and thickness at its lower edge was reduced to0.10 cm.

The stretched rubber was then exposed to an air sample containing 200ppb ozone and four photographs were taken, the first immediately andthen at intervals of 4 minutes, 8 minutes, and 2 hours.

There was essentially no frosting in the first photograph. Frosting inthe second photograph occurred at the lower or the third, where therubber piece was thinnest. Frosting in the third photograph occurred atthe lower half. Frosting in the fourth photograph occurred in the lowest⅔ of the rubber piece, indicating that frosting stopped at about thatposition and that further frosting would not take place if exposure tothe ozone-containing environment continued. These photographsdemonstrate a “front” of frosting proceeding upward from the thinnestportion of the trapezoidal rubber piece. The rate of advance of thefront is proportional to the ozone concentration, which can bedetermined by the use of a calibration.

While presently preferred embodiments have been shown of the inventiondisclosed herein, persons skilled in this art will readily appreciatethat various additional changes and modifications may be made withoutdeparting from the spirit of the invention as defined and differentiatedby the following claims.

What is claimed is:
 1. Ozone detector comprising stretched materialwhich creates microcracks therein when exposed to ozone and a standardhaving a plurality of indicia which indicate ozone concentration basedon integral intensity of light reflected or scattered from saidmaterial, cross-sectional area of said material is selected from thegroup consisting of uniform and non-uniform.
 2. Ozone detector of claim1 wherein said material is elastomeric.
 3. Ozone detector of claim 1wherein said material is selected from the group consisting of syntheticrubbers, natural rubbers, and mixtures thereof, and wherein saidstandard is a single support having a plurality of said indicia thereonwhich indicia are indicative of ozone concentration based on therelationship of increased degree of microcracking in said materialrepresenting greater ozone concentration.
 4. Ozone detector comprisingmaterial which can create microcracks therein when exposed to ozone,thickness of said material is selected from the group consisting ofuniform and non-uniform; means for stretching said material; a lightsource for projecting a light at said material in stretched condition;and means for detecting integral light intensity after it is reflectedor scattered by said material wherein signal produced by the means fordetecting is dependent on the presence of ozone in contact with saidstretched material.
 5. Ozone detector of claim 4 wherein said materialis selected from the group consisting of synthetic rubbers, naturalrubbers, and mixtures thereof.
 6. Ozone detector of claim 4 comprisingan enclosed chamber; said material disposed in said chamber; an openingin said chamber for said material to exit; and means for translatingsaid material between said opening in said chamber and a point removedfrom said chamber.
 7. Ozone detector of claim 6 including a converginglens which is adapted to receive the light reflected or scattered bysaid material and direct it to said light detecting means.
 8. Ozonedetector of claim 6 which includes means for collecting the reflected orscattered light.
 9. Ozone detector of claim 8 wherein said material isin the form of an elongated film wound on a spool in said chamber andsaid opening in said chamber includes means for restricting air fromentering into said chamber.
 10. Ozone detector of claim 9 which includesa current amplifier connected to laid light detector and a voltmeterconnected to said current amplifier.
 11. Ozone detector of claim 10including means for neutralizing ozone in said chamber so that it doesnot create microcracks in said material.
 12. Ozone detector of claim 11including a diverging lens for enlarging the light from said lightsource.
 13. Ozone detector comprising an extended material of uniformthickness but of non-uniform width which creates microcracks whenexposed to ozone and a standard having a plurality of indicia based onintegral light reflected or scattered by said material indicating ozoneconcentration based on the relationship of increased degree ofmicrocracking in said material representing a greater ozoneconcentration.
 14. Ozone detector of claim 13 wherein said material isselected from the group consisting of natural rubbers, synthetic rubbersand mixtures thereof.
 15. Ozone detector of claim 14 wherein saidmaterial is selected from the group consisting of polybutadiene, naturalrubber, and mixtures thereof.
 16. Method for detecting ozoneconcentration comprising the steps of (a) stretching a material, (b)exposing the material to a gaseous mixture containing ozone and allowingfrosting of the material to take place based on formation of microcracksdue to ozone in the gaseous mixture, (c) exposing the material to light,and (d) detecting ozone concentration in the gaseous mixture based onintegral light reflected or scattered by the microcracks in thematerial, amount of microcracking is directly related to the ozoneconcentration in the gaseous mixture.
 17. Method of claim 16 wherein thegaseous mixture contains ozone in approximate concentration of 0.001-1ppm and the material is selected from the group consisting of syntheticrubbers, natural rubbers, and mixtures thereof.
 18. Method of claim 17wherein said step of exposing the material to light is accomplished byprojecting a light emanating from a light source.
 19. Method of claim 18including the step of converging the light reflected or scattered by thematerial.
 20. Method of claim 19 wherein the material is selected fromthe group consisting of polybutadiene, natural rubber, and mixturesthereof.